Method for making a lithographic printing plate according to the silver salt diffusion transfer process

Abstract

The present invention provides a method for making a lithographic printing plate from an original containing continuous tones comprising the steps of: frequency modulation screening said original to obtain screened data image-wise exposing according to said screened data an imaging element comprising in the order given on a hydrophilic surface of a support (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing one or more silver halide emulsions being in water permeable relationship with said image receiving layer applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s), treating the imaging element to remove the layer(s) on top of said image receiving layer, thereby uncovering said silver image formed in said image receiving layer, characterized in that said aqueous alkaline solution is substantially free from thiosulphate ions and bromide ions and contains an aminoalcohol in an amount ranging from 0.1 ml to 10 ml/l aqueous alkaline solution.

I claim: 
     
       1. A method for making a lithographic printing plate from an original containing continuous tones comprising the steps of: frequency modulation screening said original to obtain screened data   image-wise exposing according to said screened data an imaging element with a scan-wise exposure using a laser or LED, said element comprising in the order given on a hydrophilic surface of a support (i) an image receiving layer containing physical development nuclei, (ii) a photosensitive layer containing one or more silver halide emulsions being in water permeable relationship with said image receiving layer   applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s),   the imaging element to remove the layer(s) on top of said image receiving layer, thereby uncovering a silver image formed in said image receiving layer, characterized in that said aqueous alkaline solution is substantially free from thiosulphate ions and bromide ions and contains an aminoalcohol in an amount ranging from 0.1 ml to 10 ml/l aqueous alkaline solution.   
     
     
       2. A method according to claim 1 wherein said aqueous alkaline solution contains less than 0.001 mole/l of inorganic bromide salts. 
     
     
       3. A method according to claim 1 wherein said aqueous alkaline solution contains less than 0.001 mole/l of thiosulphate salts. 
     
     
       4. A method according to claim 1 wherein said aminoalcohol is a primary aminoalcohol. 
     
     
       5. A method according to claim 4 wherein said primary aminoalcohol is 2-aminoethyl-aminoethanol. 
     
     
       6. A method according to claim 1 wherein said aminoalcohol is used in an amount between 0.5 and 3 ml/l aqueous alkaline solution. 
     
     
       7. A method according to claim 1 wherein said frequency modulation screening of an original proceeds according to the following steps: selecting an unprocessed image pixel of an original according to a space filling deterministic fractal curve or a randomized space filling curve and processing said unprocessed image pixel as follows: determining from the tone value of said unprocessed image pixel a reproduction value to be used for recording said image pixel on a recording medium,   calculating an error value on the basis of the difference between said tone value of said unprocessed image pixel and said reproduction value, said unprocessed image pixel thereby becoming a processed image pixel,   adding said error value to the tone value of an unprocessed image pixel and replacing said tone value with the resulting sum or alternatively distributing said error value over two or more unprocessed image pixels by replacing the tone value of each of said unprocessed image pixels to which said error value will be distributed by the sum of the tone value of the unprocessed image pixel and part of said error,   repeating the above steps until all image pixels are processed.     
     
     
       8. A method according to claim 1 wherein said frequency modulation screening of an original comprises the steps of: generating a deterministic frequency modulation screen function using a dither matrix that has a size that is a power of two, and contains threshold values that are arranged in such a fashion that, when thresholded against increasing levels of density, every halftone dot is "as far away as possible" from the halftone dots that are used to render the lower density levels;   locally randomizing said deterministic frequency modulation screen function; and   utilizing said locally-randomized deterministic frequency modulation screen function to produce the frequency modulation halftone screen.   
     
     
       9. A method according to claim 8 wherein said frequency modulation screening of an original comprises the steps of: generating a non-halftone value;   altering said non-halftone value at a higher tone resolution than the tone resolution of said original   utilizing said altered non-halftone value to produce a halftone value for the frequency modulated halftone screening.   
     
     
       10. A method according to claim 8 wherein said frequency modulation screening of an original originates a halftone screen having a plurality of halftone dots, at least some of which have a size greater than the size of a pixel of said contone original and at least some of which have a size equal to the size of a pixel of said contone original.